This invention relates to ultrasonic imaging, and more particularly to a transmit-receive transducer array and to a sector scan imaging system incorporating the array.
A steered beam B-scan ultrasonic imaging system based on phased array principles has a linear transducer array as depicted in FIG. 1., and to make a sector scan the elementary transducers are pulsed in time sequence to generate angulated acoustic beams at many angles relative to the normal at the midpoint of the array. Echoes reflected by targets in the direction of a transmitted beam arrive at the transducer elements at different times and are time delayed by different amounts before coherent summation to generate a focused echo signal. A real time cardiac scanner is described by Thurstone and VonRamm in "A New Ultrasonic Imaging Technique Employing Two-Dimensional Electronic Beam Steering", Acoustic Holography, Vol. 5, 1974, Plenum Press, N.Y., pp. 249-259.
The radiation pattern or sensitivity pattern of the transducer array during transmission and reception has, in addition to the main beams, grating side lobes displaced from the main beams at an angle that is dependent upon the spacing between elements. As the interelement spacing decreases the side lobe structure moves farther away from the main beam; overall sensitivity is the product of the transmit and receive array functions. A known phased array has low side lobe levels because the elements, which are used for both transmit and receive, are spaced uniformly at half-wavelength intervals. Resolution and artifact levels, however, are a function of maximum aperture, and to achieve reasonable resolution at the half-wavelength spacing it is necessary to have a large number of elements and transmitting and receiving channels.
The problem to be solved is that when the cost of a system is proportional to the number of transducer elements or channels and when the resolution and artifact levels are a function of maximum aperture, wide transducer spacing gives the best resolution per unit cost. But the side lobe structure moves closer and closer to the main beam and becomes objectionable when the interelement spacing is increased. In this case targets in the direction of the side lobe as well as the main beam may generate echoes which are received and contribute to the focused echo signal. What is wanted is to maintain the cost-effective use of transducers while eliminating degradation of the image caused by unwanted side lobes. Another important consideration for a cardiac scanner is that the physical size of the array should be small because it is placed between or under the ribs when viewing the heart.